Method of locating interstrand short circuits in stranded armature bars



March 27, 1951 2,546,732

R. F. EDGAR METHOD OF LOCATING INTERSTRAND SHORT CIRCUITS IN STRANDED ARMATURE BARS Filed Feb. 1, 1950 2 Sheets-Sheet l OSCILLATOR.

J" 0 Q Q o a o 7 0 Inventor":

T207231; F. Edgar,

His Attorney.

March 27, 1951 2,546,732

R. F. EDGAR METHOD OF LOCATING INTERSTRAND SHORT CIRCUITS IN STRANDED ARMATURE BARS Filed Feb. 1, 1950 2 Sheets-Sheet 2 Fig.5.

F is. 6.

a DISTHNCE ACEOSS HRMHTURE BWR- Inventor: Robew t F. E g 61*,

His Attorney.

Patented Mar. 27, 1951 METHOD OF LOCATING INTERSTRAND SHORT CIRCUITS IN STRANDED ARMA- TURE BARS Robert F. Edgar, Pattersonville, N. Y., assignor to General Electric Company, a corporation of New York Application February 1, 1950, Serial No. 141,808

chines are stranded to reduce eddy currents within the bar. Light insulation separates the strands, and heavy insulation surrounds. the entire bar. The strands are soldered together at the ends so that each bar is a stranded con ductor. If a short circuit exists between strands,

eddy currents may flow through the shorted strands suificient to cause heating at the short circuit which may eventually damage the insulation of the armature bar and cause failure of the dynamo-electric machine. Therefore, it is important to locate the interstrand short circuits before such trouble develops.

In the manufacture of stranded armature bars, it is customary to test the bar for interstrand short circuits before the strands are soldered together at their ends by applying voltage between each strand and all of the other strands. When it is discovered in this way that two strands are shorted together somewhere 'in the bar, it is desirable to have some method for quickly locating the exact position of the short circuit Within the bar without further damaging the insulation. If the short circuit can be so located, it may be possible to open the bar at this point and correct the trouble, and thus avoid scrapping the defective bar.

An object of this invention is to provide improved means for locating an interstrand short circuit within an armature bar without further damage to the insulation.

After the initial test for interstand short circuits, the ends of the strands are soldered together and the heavy outer insulation is applied to the bar. During this process, additional interstrand short circuits may develop.

Another object of this invention is to provide means for detecting and locating short circuited strands within an armature bar after the strands are soldered together at their ends.

Other objects and advantages will appear as the description proceeds. The features of this invention which are believed to be novel and patentable are pointed out in the claim which forms a part of this specification.

For a better understanding of the invention, reference is made in the following description to the accompanying drawings, in which Fig. 1 is a plan view of a portion of an armature bar and.

apparatus for locating interstrand short circuits Within the bar; Fig. 2 is a section along the line 1 Claim. (Cl. 175183) 22 of Fig. 1; Fig. 3 is anelevation showing the relative position between the armature bar and the test apparatus at a later stage in the location of a short circuit; Fig. 4 is a schematic diagram of apparatus for detecting and locating interstrand short circuits after the ends of the strands have been soldered together; Fig. 5 is a partial section along the line 55 of Fig. 4;, drawn to a larger scale; and Fig. 6 is a graph of voltage variations hereinafter more fully explained.

Referring now to Figs. 1 and 2, a large armature bar I comprises a plurality of strands 2 which are insulated one from another by insulation 3. Assume that strands 2' and 2 have been found to be short circuited somewhere within the bar. It is desired that this short circuit be located so that the bar may be repaired. Location of the short circuit is complicated by the fact that the strands do not run straight through the bar, but are transposed to reduce circulating currents.

An oscillator 4 is connected to apply alternating current to the short circuited strands through a current limiting resistor 4. The oscillator may have an output of volts at 2000 cycles per second, for example. I Current limiting resistor 4 may be in the order of 2000 ohms. With this arrangement', the maximum power which can be supplied to the short circuit is less than 2 watts, which is not sufficient to produce further damage in the insulation.

A magnetic member 5 is slightly longer than the width of the armature bar, as shown. At each end of member 5 and perpendicular thereto is a short leg 6, also of magnetic material. Member 5 and legs 6 form a U-shaped magnetic core adapted to fit loosely over the armature bar, as illustrated in Fig. 2. Upon each leg 6 there is a search coil winding 1. Attached to member 5 by bracket 8 is an electrical indicating instrument 9. A handle in may be provided for convenience in moving the magnetic core along the length of the armature bar.

Windings l are connected in series bucking relation to the input of an amplifier H. The output of amplifier I I is connected toindicating instrument 9, so that the indicating instrument reads the difference between the values of voltages induced in coil 1. The amplifier may conviently be located in a separate case and connected to the search coils and to the indicating instrument throu h-suitable cables. Since coils l are connected in series bucking relationship, any magnetic flux which travels the entire length of the U-shaped core induces equal and opposite voltages in the two search coils, and, therefore,

armature bar.

Referring to Fig. 2, assume that the instan taneous current in strand 2 is flowing into the drawing, as indicated by the arrow tail (-1-) and that the instantaneous current in strand 2" is flowing out of the drawing, as indicated by the arrow head (6). netic flux along the broken lines Hand [3. It can be observed that this flux flows into both ends of the U-shaped magnetic. core, and out This current produces mag- 4 across the slot of fixture l5 causes circulating currents to flow through the short circuited strands. The instantaneous direction of this current in strand 23 may be into the drawing, as indicated by the arrow tail (-1-), and the current in strand 24 may beout of the drawing, as indicated by the I arrow head (0)- These currents produce magnetic flux along the paths indicated by broken lines and 26. When the center leg of core 20 is between the two short circuited strands, as illustrated in Fig. 5, the flux produced by circulating short circuit currents'travels in opposite directions through windings 2| and 22, so that the voltages induced in the two windings by this flux add together and produce a net output voltage from the search coil. The magnetic fiux produced by circulating near the center of the core, thereby inducing in I windings 7 voltages which add together to pro-v duce an indication at instrument 9.

Theshort circuit between strands 2' and 2 is located by placing member 5 across the armature bar at the end of the bar to which oscillator 4 is connected. The current through these short circuited strands produces an indication at instrument 9, as has been explained. Member 5 is moved along the length of the bar until the indication provided by. instrument 9 suddenly decreases. This decrease in indication shows that short circuit current is no longer flowing through the bar immediately below member 5. Therefore, the location of .the interstrand short circuit must be somewhere in. the bar below member 5 in the position at which the sudden decrease in instrument reading occurs.

To locate the short circuit across the width of the armature bar, member Sis lifted'from the bar and the surface of the bar is explored with one of the coils i, as illustrated in Fig. 3. Maximum indication of instrument 9 occurs when the coil 1 is directly over the interstrand short. The armature bar may then be opened at this point and the trouble repaired.

After the ends of -the-strands have been soldered together, the detection and location of short circuits maybe accomplished in the following manner: Referring to Fig. 4, a completed armature bar [4, which is to be tested, is placed within the slot of a slotted. magneticfix-ture l5. Fixture [5 may be made of stacked laminations to simulate an armature slot .inf'a dynamoelectric machine. Alternating magnetic flux... is provided across the slotof fixture [5 by a second armature bar 16, placed at the bottom-of the slot, through which SDOamperes -or more of alternating current is provided .throughconnections ll. It will be appreciated that other means, such as a winding,..may,be employed to provide the magnetic flux across the slot. The two ends of the armature bar M are explored simultaneously with similar test coils l8 and I9, respectively. The construction of the test coils is more clearly shown inFig. 5. I

Referring now to Fig. 5, test coil 18 comprises an E-shaped magnetic core 20 and two windings 2! and 22 upon opposite sides of the magnetic core. Windings 2 I and 22 are connected in series bucking relation so that any external magnetic 'field, such as the magnetic field produced across the slot of fixture l5, induces equal and opposed voltages in the two windings. These voltages cancel, so that the net output of search'coil I8 is zero.

Assume that an interstrand short circuit exists between strands 23 and 24, somewhere within the The alternating magnetic flux short circuit.

short circuit current is in phase quadrature to the flux across the slot of member 25, and, therefore, the voltage induced in the search coil by the short circuit currents is in phase with the flux across the slot.

Due to imperfect transportation of the strands, or to other reasons, circulating currents may be produced in strands which are not short circuited. These circulating currents also produce magnetic flux which induces in the search coils voltages which are in phase with the magnetic flux across the slotted member l5. These circulating currents might produce shortcircuit indications if only one search coil were used. Such voltage indications are eliminated by using two search coils l8 and I9, as shown in. Fig. 4, connected in series bucking relation. The two search coils l8 and [9 are moved simultaneously by equal amounts across the respective ends of armature bar i l, so that the two search coils are. always adjacent to the opposite ends of the same strands. Circulating currents in strands which are not short circuited must flow in the same direction at both ends of the armature bar. Therefore, the voltages produced in the search coils by these circulating currents are opposed and cancel out. On the other hand, circulating currents in short circuited strands flow in opposite directions at the respective ends of the armature bar. Therefore, these currents cause voltages to be induced in the two search coils which add and produce an indication of a A compensating network comprising capacitors 21 and 28 and tapped resistors 29, 30 and 3,l, connected as shown, may be provided to balance any dissimilarities in the characteristics of the two, search coils.

The net voltage from the two search coils .is applied to a phase selective indicating circuit which may comprise a'triode 32, rectifiers 33 and 34, and voltmeter 35. The net voltage from the search coils is applied between grid and cathode of triode 32 through connections 36 and 3'! and capacitor 3 8. A grid leak resistor 39 and a bias voltage source 46 may be, provided to maintain the proper bias potential'at the grid oftriode The plate of .triode 32 is connected to the center of a. voltage.dividercomprising resistors 141L 42 and 4-3 connected in series between the cathodes or rectifiers 33 and 34. Voltmeter35is also. connected between the cathodes of the two rectifiers. The anodes of the rectifiers are connected to opposite ends-of the secondary winding The primary winding 0f phase adjusting network comprising capacitor 46 and variable I resistor 41. The primary winding of transformer 45 may be connected through another transformer 48 to the alternating current supply connections 11. Transformer 48 is a current transformer having a loading resistor 49 connected across its secondary winding. The current through resistor 49, and the voltage across this resistor are in-phase with the current in the bar It and, therefore, in-phase with the flux across the gap of fixture l5.

Rectifiers 33 and 34 conduct current alternately, as in a conventional full-wave rectifying circuit. When the potential at the grid of triode 32 is the same during both half cycles of the supply current, rectifiers 33 and 34 conduct equal amounts of current, and the indication of voltmeter 35 is zero. in the triode grid potential during the two half cycles, the rectifiers conduct unequal currents, and the resulting difference between the respective voltage drops across resistors M and 43 causes a voltage indication by the voltmeter. voltmeter 35 thus indicates the value of inphase alternating potential applied to the grid of triode 32, and furthermore the polarity of the voltmeter indication shows the phase reversals of this potential. Voltages which are in phase quadrature to the supply current produce no indication at voltmeter 35. The phase of current supplied to the rectifiers may be adjusted for best operation by adjusting the value of resistor 41.

Curve 5!], Fig. 6 is a graphical representation of the voltmeter indications plotted as a function of the position of search coils I-8 and I9 across the Width of armature bar l4. At point 5| a large negative indication of the voltmeter is obtained. Such an indication shows that at this point the search coils are adjacent to short circuited strands in the armature bar. These strands may be marked, and the armature bar then removed from the test fixture. The solder at the ends of the short circuited strands may When there is a difference be removed, and the short circuit may then be located within the bar in the manner described in connection with Figs. 1, 2, and 3.

Having described the principle of this invention and the best mode in which I have contemplated applying that principle, I wish it to be understood that the examples described are illustrative only, and that other means can be employed without departing from the true scope of the invention.

What I claim as new and desire to secureby Letters Patent of the United States is:

The method of locating interstrand short circuits in stranded armature bars, which comprises placing the armature bar within a slot in a slotted magnetic fixture so that the ends of the bar extend beyond the ends of saidjjfixture, producing alternating magnetic flux across the slot in said fixture, whereby circulating electric currents are produced through short j circuited strands of the armature bar, exploring both ends of the armature bar simultaneously with search coils, so that circulating currents through short circuited strands induce voltages in the search coils when adjacent to such strands which are in phase with the magnetic flux across said slot, said search coils being moved simultaneously by equal amounts across the width of the armature bar so that the respective coils are always adja cent to opposite ends of the same strands, and detecting the difierence between values of such voltages induced in the respective coils.

ROBERT F. EDGAR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 925,594 Pennell et al June 22, 1909 1,297,929 Taylor Mar. 18. 1919 

